Endress+Hauser, the assignee of the present application, works in the field of industrial automation- and process control technology and manufactures industrial measuring devices, also referred to as field devices. It sells these field devices e.g. for fill level determination of a medium in a container. Such devices include, among others, devices sold under the marks, MICROPILOT, LEVELFLEX and DELTAPILOT.
A known measuring method, among a large number of measuring methods for ascertaining fill level in a container, is the travel time, measuring method. In the case of this measuring method, for example, microwaves are transmitted via an antenna apparatus and echo waves reflected on the surface of the medium detected, wherein the travel time of the measurement signal is a measure for the distance traveled. From half the travel time, accordingly, fill level of the medium in a container can be ascertained. The echo curve represents, in such case, the total course of the signal as a function of time, wherein each measured value of the echo curve corresponds to the amplitude of an echo signal reflected on a surface located at a certain distance. The travel time, measuring method is essentially divided into two methods of ascertainment: In the case of the time-difference method, the time, which a broadband signal pulse requires for a traveled path, is ascertained. In the case of the frequency difference method (FMCW—Frequency-Modulated Continuous Wave), the transmitted, frequency modulated, high-frequency signal is compared with the reflected, received, frequency modulated, high-frequency signal. In the following, no limitation is intended as to which particular method of measurement is used.
In the case of certain process applications, the measuring devices, or their sensor elements, are exposed to extreme conditions, such as e.g. high temperatures, high pressures and/or chemically aggressive substances. Especially, microwave fill level measuring devices have temperature- and/or pressure-sensitive components, such as, for example, the measuring device electronics and transmitting- and/or receiving elements for the microwaves. Moreover, the radiating characteristics of the antennas of the microwave fill level measuring devices are changed by accretions of medium.
In order to protect the measuring electronics in the measurement transmitter and the microwave signal coupling structures against high temperatures, high pressures and aggressive chemical materials, as well as to hold the radiating characteristics of the antennas constant, sensitive elements of the sensor elements, respectively the antennas, are hermetically sealed on the process-side with process isolation elements. Moreover, by joining a hermetically sealed, process isolating element into the hollow conductor of a horn antenna, the greatest possible safety is assured, since, due to a second “safety element”, the process, with an isolation of the modular measurement-active parts, such as e.g. an in-coupling unit/exciter element or the measuring device electronics, from the measurement passive parts, such as e.g. the antenna, still remains sealed, should maintenance or repair of the fill-level measuring device become necessary.
Described in WO 2003/046491 A1 is an antenna for fill level measurement, which is protected against aggressive chemical media and high temperatures by means of at least partial filling with a dielectric material or a disk or washer shaped, dielectric element.
U.S. Pat. No. 5,115,218 B2 discloses a microwaves transmissive, process isolating element, whose conical formation uses Brewster's angle, which is the angle for total transmission of the radiation energy through a dielectric medium.
Shown in WO 2000/29819 A1 is a microwaves process isolation window for a bypass-pipe that has a conical formation and is hermetically sealed via O-rings between process connection nozzle on the container and device connection nozzle.
Presented in DE 10 2007 026 389 A1 is a high temperature process isolation, which has an attenuating, or damping, element, which improves measurements in the vicinity of the antenna by reducing ringing.
Disadvantageous in the case of the process isolation elements shown in the state of the art is that the synthetic materials of the process isolating element and the seal elastomers are not durable in the temperature range >200° C. and, with rising temperature, very rapidly age- and, as a result thereof, have leakage problems.